Weegy: In May of 1915, German submarines sank the RMS Lusitania, an ocean liner (although it had previously been used as a warship, and was later found to have been carrying munitions when it was sunk). [ Over 100 Americans were killed, and this is generally seen as being the point where the US started to lean towards the side of the Allies, despite Germany agreeing to cease unrestricted submarine warfare.
By the end of 1916, Germany was in dire straits. They had been blockaded by the massive British Navy since the war began, and it was becoming a major problem. Germany decided they needed to break their promise and restart unrestricted submarine warfare in an attempt to curtail the British blockade. They also realized that the US would be angry.
So in an effort to help neutralize the American threat, Germany tried to secretly arrange an alliance with Mexico. Germany wanted Mexico to join the war on the side of the Central Powers, promising to support the Mexicans, and if the Central Powers won the war, Germany would see to it that Texas, New Mexico and Arizona were given back to Mexico. However, British spies were able to intercept the message and deliver it to the Americans.
Thus, by January 1917, the US was already leaning towards the Allied side. Then they found out that the Germans were going to break their pledge and start sinking ships unabated again, and not only that, but Germany had tried to get Mexico in on it (Mexico did get the message and refused the offer- Germany had no way of actually supporting Mexico, and Mexico was in the middle of its own civil war, so it couldn't hope to fight and win a war with the US too). The US President was furious and shortly thereafter, convinced Congress to declare war (which it did).
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